Piezoelectric ceramic compositions

ABSTRACT

A PIEZOLECTRIC CERAMIC COMPOSITION WHICH CONSISTS ESSENTIALLY OF THE TERNARY SOLID SOLUTION   PB(CO1/3, NB2/3)X, ZRY, TIZ)O3   WHERE X+Y+Z=1 AND THE COMPOSITION LIES WITHIN THE AREA A-B-C-D-E-F OF THE TRIANGULAR COMPSOITION DIAGRAM OF THE DRAWING, WHEREIN AN OXIDE OF TANTLUM OR THORIUM IS ADDED WITH THE RANGE OF FROM ABOUT 0.2 TO ABOUT 1.0 WEIGHT PERCENT OF THE COMPOSITION TO IMPROVE THE ELECTROMECHANICAL COUPLING COEFFICIENT AND THE MECHANICAL QUALITY FACTOR OF THE COMPOSITION.

3,700,596 PIEZOELECTRIC CERAMIC COMPOSITIONS Fumio Naito, Toshiyuk Kudo, and Masanobu Kubosaki, Hirakata, Osaka, Japan, assignors to Sanyo Electric Co., Ltd., Moriguchi City, Osaka, Japan Filed .lluly 31, 1968, Ser. No. 748,993 Claims priority, application Japan, Aug. 1, 1967, 42/49,453 Int. Cl. C04b 35/46, 35/48, 35/50 U.S. Cl. 252-623 3 Claims ABSTRACT F THE DISCLOSURE A piezoelectric ceramic composition which consists essentially of the ternary solid solution Pb[(C01/a, Nba/20x, Zfy, Tizlos where x+y|z=1 and the composition lies within the area ABC-D-E-F of the triangular composition diagram of the drawing, wherein an oxide of tantalum or thorium is added with the range of from about 0.2 to about 1.0 weight percent of the composition to improve the electromechanical coupling coetiicient and the mechanical quality factor of the composition.

BACKGROUND OF THE INVENTION This invention relates to improved piezoelectric ceramic compositions, particularly to ceramic compositions consisting essentially of the ternary solid solution Among the conventionally widely used piezoelectric ceramic materials are those which respectively have lead zirconate-lead ttanate or barium ttanate as their principal components. Various improvements have heretofore been made in these materials. For example, it has been proposed that part of Pb of lead zirconate-lead ttanate is replaced by a metal ion such as barium, strontium and calcium. Another attempt for improvement was to add various metal oxides to the known piezoelectric ceramic materials. 'I'hese attempts aimed at enhancement of the piezoelectric properties and improvement of the sinterability of the materials. These improved materials have their respective merits and demerits. For example, the ceramics consisting essentially of lead zirconate-lead ttanate are superior to barium ttanate type ceramics in the electromechanical coupling coeliicient and the stability of the resonant frequency with operating temperature but they are disadvantageous in that since a considerably high sintering temperature (usually 1300 C. or above) is required, the evaporation of lead oxide `during the sintering operation makes it diicult to manufacture desired sintered products in addition to the fact that a slight change in composition causes a great change in piezoelectric properties so that uniform products can hardly be obtained. A further disadvantage of the lead zirconate-lead ttanate type ceramics is that a high electric field such as 40-50 kv./cm. is required for polarization treatment.

It is, therefore, the fundamental object of the invention to provide new and improved piezoelectric ceramic materials which, over a relatively wide range of composition, have a high electromechanical coupling coeicient and/ or a good stability in the resonant frequency with operating temperature and are easy to sinter and to polarize.

nited States Fatent W 3,700,596 Patented Oct. 24, 1972 ice SUMMARY 0F INVENTION The piezoelectric ceramic composition according to the invention is of a ternary solid solution consisting essen- Of Pb(CO1/3Nb2/3)O3, and Th tefnary solid solution may be expressed by the following general formula:

is Within the range of 2-60, the mole percentage of PbZrO3 is within the range of 0-80 and the mole percentage of PbTiO3 is within the range of 0-60.

Piezoelectric properties of the above mentioned ceramic composition can be improved by adding thereto various impurities. The impurities which are used therefor are selected from the group consisting of oxides, of manganese, tantalum, thorium, zinc and chromium. The amount of any impurity to be added should be up to 3 weight percent of the total quantity of the ceramic composition.

BRIEF DESCRIPTION OF THEl DRAWING The single ligure of the drawing represents the triangular compositional diagram of the ternary system utilized in the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS To obtain piezoelectric ceramic materials according to the invention, starting materials including, PbO', COO, Nb205, ZrOz and Ti02 are intimately mixed for 10-15 hours in a ball mill with water so as to have a relatively uniform grain size. After drying, the mixture is pulverized into a finely divided form so as to make it as homogeneous as possible. The finely divided mixture is molded at a pressure of 1 ton/ern.2 into a desirable form. The block thus obtained is calcined at S50-900 C. for two hours, and pulverized again in the ball mill. The pulverized material is then molded at a pressure of 1 ton/cm.2 into compacts in the form of a `disk having a diameter of about 8 mm. and a thickness of about 1 mm. meter. The compacts are sintered at a temperature within the range of U50-1300 C. for about one hour. The sintering temperature depends on the molar ratio of the composition. The sintering products are polished on the opposite surfaces to the thickness of about 0.4 mm. The polished disk is coated on the opposite surfaces with silver paint and tired at 600 C. to form electrodes. The products are then as the last step, subjected to a polarization treatment which may be carried out by immersing the products in silicon oil at C. and applying a D C. electric eld of 15 kv./cm. thereto.

Forty six specimens having different composition ratios were prepared according to the above mentioned manner. Various properties of those specimens have been measured and are listed in Table 1. In Table 1 each of the values of various properties is an average value for the specimens having the same composition. kp indicates the radial electromechanical coupling coefficient and T.K(f) indicates the temperature change in resonant frequency which is calculated from the following formula:

fr (60C.) -fr (-20C.)

3 where fr(60 C.), fr(-20 C.) and fr(20 C.) are resonant frequency at 60 C., 20 C. and 20 C., respectively. The measured temperature range is I-2() C. to i60 C.

It will be understood from Table 1 that in any of the specimens listed therein, since the sintering can be carried out at a relatively low temperature such as below l300 C., there is no fear of evaporation of lead oxide so that a simpler apparatus may be used for the sintering and materials of a higher density which ensures a superior wet-proofness may be obtained. In addition, all of those specimens listed in Table l can be polarized by application such a relatively low D.C. iield such as 15 kv./cm. This enables to simplify the apparatus for polarization.

The drawing represents the triangular compositional diagram of the ternary solid solution,

Pb (CO1/3Nb2/3) O3'PbZ1-O3-PbTiO3 in which the specimen numbers in Table 1 are shown.

' Compositions defined by the polygonal region ABCDEF have common characteristics as described above and shown in detail in Table 1. The mole percentage each of the three components of each vertices ABCDEF is given as follows:

Pb (C01/aNb2/3)03 PbZrO; PbTiO;

Any of the compositions within the polygonal region ABCDEF consists of 2 60 mole percent of -80 mole percent of PbZrO3 and 0-60 mole percent of PbTiO3. The positions of B, C and E are identical with the positions of the specimens having Example Nos. 4l, 46 and 12, respectively.

Within the polygonal region GHIJK in the drawing, ceramic products exhibit a particularly excellent electromechanical coupling coefliciert of approximately 40% or higher. The mole percentage each of the three components of each vertices vGHIJK is given as follows:

Pb (C0i/3Nb2/3)03 PbZrO3 PbTiO The positions of G, H, I, J and K are identical with the positions of the specimens bearing Example Nos. 7, 16, 27, 24 and 15, respectively. The ceramic products having a composition within the polygonal region GHIJK in FIG. 1 are highly valuable as piezoelectric elements for use in pickups, microphones, vibrators, vibration gauges and ignition elements because of their highly responsive piezoelectric elect.

With the polygonal region LBCMNOP in FIG. 1, ceramic products show a small change in resonant frequency less than 90X 10-6/ C. within the temperature range of -20 C. to 60 C. The mole percentage each 4 of the three components of each vertices LBCMNOP is given as follows:

The positions of L, B, C, M, N and O are identical with the positions of the specimens bearing .Example Nos. 2, 41, 46, 43, 38 and 26, respectively. The ceramic products having a composition within the polygonal region LBCMNOP in the drawing are highly valuable as piezoelectric ceramic materials such as ceramic lilters for radio or TV use.

Piezoelectric and dielectric properties of the ceramic products having a basic composition described in the above may be improved by adding thereto various impurities. Among the impurities which are useful therefor, there may be mentioned Mn02, TagO, ThO2, ZnO and Cr203. These impurities may be added either solely or jointly but the amount should be up to three weight percent preferably 0.5 to 1.0 percent by weight of the total quantity of the ceramic product. Some examples for addition of various impurities are given hereinbelow.

Table 2 shows the dielectric, piezoelectric and ceramic properties of the present ceramic compositions with or without Mn02 in an amount of up to 3.0 percent by weight. `It will be apparently understood that the addition of Mn02 to the basic compositions listed in and sampled from Table 1 reduces their resonant resistance R to a substantial extent and remarkably increases the mechanical quality factor QM.

The addition of ZnO is also elective to reduce the resonant resistance R and to increase the mechanical quality factor QM. Table 3 shows the dielectric, piezoelectric and ceramic properties of the ceramic compositions with or without ZnO in an amount of up to 3.0 percent by weight.

'I'he addition of Ta205 is eifective to increase the electromechanical coupling coeicient kp, the mechanical quality factor QM and the dielectric constant e. Table 4 shows the dielectric, piezoelectric and ceramic properties of the ceramic compositions with or without Ta2O5 in an amount of up to 3.0 percent by Weight.

The addition of Th02 is also elfective to increase the electromechanical coupling coelicient kp, the mechanical quality factor QM and the dielectric constant e. Table 5 shows the dielectric, piezoelectric and ceramic properties of the ceramic compositions with or without ThOz in an amount of up to 3.0 percent by weight.

The addition of Cr203 is effective to increase the electromechanical coupling coeieient kp, the mechanical quality factor QM and the dielectric constant e as Well as in case of addition of Ta2O5 or Th02. Table 6 shows the dielectric, piezoelectric and ceramic properties of the ceramic compositions with or without Cr203 in an amount of up to 3.0 percent by weight.

It will be understood from the foregoing that the ternary solid solution according to the present invention and the solid solution including an appropriate additive form excellent piezoelectric ceramic bodies. While there have been described what at present are believed to be the preferred embodiments of this invention, it will be obvious that various changes and modilications can Vbe made therein without depart-ing from the invention, and it is aimed, therefore, to cover 'in the :appended iclaims all such changes and modifications `as fall within the true spirit and scope of the invention.

TABLE l mmc D 1 s Example No.

0.296L32534l73 5535127.52029om 785 06878394022 21133341222.1134@3214423444421 333w323333 3312 53545 50555002 805508570250508 0468003500 453 .7MH1 .4cm IBMWQMAAG .22334445233MOM3333463252M 52 n wm. .D A.

2 0 00 255555wwmmmm2wm22m TABLE 2 Electromechanical Mole percentage of basic composition Additive Dielectric PbZrOa PbTiO; M1102 Sintering (Weight percent) coupling Mechanical Resonant cocfcient,

Pb (ComNbg/a) O: (mole (mole Example No. (mole percent) percent) percent) 0250 O QL ass 1, co2 1, 289

TABLE 2-C'ontinued Electromechanical Mole percentage of basic composition i Additlve PbZrOa PbTiO; MnOa Sintering Dielectric coupling Mechanical Resonant Pb (Co uxNbz/DO: (mole (mole (weight tem Density constant coecient, quality resistance, Example No. (mole percent) percent) percent) percent) C.) (gJcmJ) f kp (percent) factor, QM R (il) mman TABLE 3 Electromechanical Mechanical coupling quality coeiicient Resonant factor resistant temp. Density Dielectric CJ Example No. Pb(Co1/aNb2/a)05 PbZrOa PbTiOa (gJcm) constante kp (percent) Mean TABLE 4 Additive Mechanical Mole percentage of basic composition TazOs Sintering quality (weight temp. factor Example No. Pb(Coxi1Nbz/;) 0; PbZrOz PbTOa percent) C QM TABLE 5 Mole percentage of basic composition Sintering Mechanical Additive temp. quality Example No. Pb(Co1/'Nr2l3) 0| PbZrO; PvTlO; ThOz C.) QM

TABLE 5-Continued Electromechanical Mole percentage of basic composition Siutering Dielectric coupling Mechanical Resonant Additive temp. Density constant coefiicicnt quality resistant Pb(Co;/3Nr2/3)O3 PbZrO3 PvTlOa ThOg C.) (g/.em.3) e kp (percent) Qn R (S2) TABLE 6 Electro- Additive mechanical Mechanical CrgOg Sintering Dielectric coupling quality Resonant (weight temp Density constant eoetcient kp factor resistant PbZrO; PbTiOa percent) C.) (g./om.3) e (percent) QM R (S2) Mole percentage of basic composition Pb(C0|/3Nb2l3) 0 awing,

ion diagram of the dr and further containing an additive selected from the group consisting of oxides of tantalum and thorium in an What is claimed is:

1. A piezoelectric ceramic material consisting essentially of a composition of the ternary solid solution exthe triangular composit 2. A piezoelectric ceramic material as in claim 1 wherein the composition lies within the area G-H-I-J -K in the triangular composition diagram of the drawing.

pressed by the general formula Pb[Co1/3, Nb2/3)X, Zry, Tiz] O3, where x+y+z==1, Within the area A-B-C-D-E-F in amount sufficient to increase both the electromechanical coupling coefficient and mechanical quality factor of said composition, said amount being within the range from about 0.2 to about 1.0 weight percent. 

